Eli5 reposts become worse in quality

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Why are pictures that are reposted often, eventually become worse quality when screenshots take the same image

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Every time you upload an image, it undergoes compression to save space on the servers. This compression is lossy, as in the quality of the image degrades when compressed

When you take a screenshot, you are taking it of an image that has already had lossy compression. When you upload it again, it undergoes more compression, reducing the quality further

The more you do it, the lower the quality

There is a good answer but it’s leaving out some detail. Which is kind of ironic considering the detail it leaves out.

There are different “formats” for how computers store images.

Some of them, like “bitmaps”, take up a LOT of space but always represent the same image perfectly. These are called “lossless” image formats. If people use lossless formats, it doesn’t matter how many times an image is manipulated or captured, it will have the same quality.

JPEG became the king of the internet image formats because it is a “lossy” format. That means, oversimplified, it uses math to find parts of the image we won’t notice are missing and deletes them. There is a “quality” option in its algorithm that makes it more or less picky about how much it deletes. But even if you save at very high quality, a tiny bit of information gets lost every time a JPEG is saved. That’s why they get worse and worse as they are reposted.

PNG is another format that is popular today. It is lossless like bitmaps, but does more work to take up less space so it’s kind of an in-between in that it can provide high quality in a lossless way but not require the same amount of space as JPEGs. GIF is another popular format that’s in-between: it’s “lossy” because it has a very limited number of colors it can use so the first time you save it, the quality gets a lot worse. But it’s “lossless” because after you take that one big hit to quality, saving it again won’t make it worse.

A lot of screenshot tools save to JPEG because people are just sort of used to seeing “.jpg” on the end of pictures. Even a lot of cameras use JPEG, though the really good ones usually have an option to let people ask them to save images as “RAW” instead. As you can guess, “RAW” is a lossless format that uses more space but loses no quality.

There are some other good formats with different balances between quality and file size, but it’s REALLY hard to fight against 50 years of internet traditions and get browsers/etc. to support new formats. For example, it took something like 10 years before all of PNG’s features were supported in the main browsers.

The screenshot tools could save to PNG and probably even have that feature, but most people don’t really care about this and click the smallest number of buttons possible to get their meme posted.